1. Field of the Invention
The present invention relates to polypropylene resin pre-expanded beads for obtaining polypropylene resin expansion-molded products in a mold, a manufacturing method of said beads and an expansion-molding method using said beads, more particularly, it pertains to polypropylene resin pre-expanded beads which lend themselves to cutdown on costs per unit weight for storing and transporting pre-expanded beads, manufacturing methods thereof and an expansion-molding method therewith.
2. Description of the Prior Art
With polyolefin resins including polypropylene resins, generally being inferior in foaming agent holding property, it is difficult, unlike with styrene resins, to keep in store the base material resin pellets impregnated with any foaming agent, in their unexpanded state, to transport them to the desired destination in the desired time and, then, to produce pre-expanded beads by use of a simple pre-expanding machine in which they are heated with steam, etc.
Accordingly, as a method of obtaining polyolefin resin pre-expanded beads, a method of impregnating the base material resin pellets with a foaming agent and thereafter, quickly expanding them at the desired expansion ratio (for example, Patent Publication No. 56-1344 and Patent Laid-Open No. 58-197027), etc., are employed. Now, speaking of one of characteristic features of bulk density of polyolefin resin pre-expanded beads obtained in this way, when, for example, a cross-linked or non-cross-linked polyethylene resin is used as the base material, its bulk density, after it has been left intact at the normal temperature and at atmospheric pressure, becomes higher than that just after its expansion, i.e., it has shrinking tendency. In contrast, with a polypropylene resin as the base material, as according to this invention, its bulk density just after its expansion will barely undergo change, even after it has been left intact at the normal temperature and at atmospheric pressure. Thus, according to the conventional technique, polypropylene resin pre-expanded beads are once kept in store with their bulk density unaltered they had immediately after their expansion, and transported to any desired destination in any desired time, and subjected to an expansion-molding in a mold.
When expansion-molded products are produced rapidly by use of pre-expanded beads, it is necessary to once keep the pre-expanded beads in store, so that the necessary amount may be transported in the desired time to the places where an expansion-molding is to be done therewith. In view of the fact that customers for expansion-molded products exist in wide areas, the places where the expansion-molding is to be done with the beads should desirably be provided in wide areas. Accordingly, it is not a rare occasion that pre-expanded beads are transported to remote places. In this situation, to keep in store pre-expanded beads generally having bulk density of 0.0090-0.0930 g/cc until the time when their use is necessary and, then, to transport them to remote places in the desired time, vast storing areas and large capacity transporting means are necessary, and as a consequence, the storing and transporting costs per unit weight of bead will become invariably enormous.